Method for generating hypermutable plants

ABSTRACT

Blockade of mismatch repair in a plant can lead to hypermutation and a new genotype and/or phenotype. One approach used to generate hypermutable plants is through the expression of dominant negative alleles of mismatch repair genes in transgenic plants or derived cells. By introducing these genes into cells and transgenic plants, new cell lines and plant varieties with novel and useful properties can be prepared more efficiently than by relying on the natural rate of mutation. Moreover, methods to inhibit the expression and activity of endogenous plant MMR genes and their encoded products are also useful to generate hypermutable plants.

This application claims the benefit of provisional application Ser. No.60/183,333, filed Feb. 18, 2000.

TECHNICAL FIELD OF THE INVENTION

The invention is related to the area of mismatch repair genes. Inparticular it is related to the field of mutagenesis.

BACKGROUND OF THE INVENTION

Within the past four years, the genetic cause of the HereditaryNonpolyposis Colorectal Cancer Syndrome (HNPCC), also known as Lynchsyndrome II, has been ascertained for the majority of kindreds affectedwith the disease (1). The molecular basis of HNPCC involves geneticinstability resulting from defective mismatch repair (MMR). To date, sixgenes have been identified in humans that encode proteins which appearto participate in the MMR process, including the mutS homologs GTBP,hMSH2, and hMSH3 and the mutL homologs hMLH1, hPMS1, and hPMS2 (2-7).Germline mutations in four of these genes (hMSH2, hMLH1, hPMS1, andhPMS2) have been identified in HNPCC kindreds (2-7). Though the mutatordefect that arises from the MMR deficiency can affect any DNA sequence,microsatellite sequences are particularly sensitive to MMR abnormalities(8,9). In addition to its occurrence in virtually all tumors arising inHNPCC patients, Microsatellite Instability (MI) is found in a smallfraction of sporadic tumors with distinctive molecular and phenotypicproperties (10).

HNPCC is inherited in an autosomal dominant fashion, so that the normalcells of affected family members contain one mutant allele of therelevant MMR gene (inherited from an affected parent) and one wild-typeallele (inherited from the unaffected parent). During the early stagesof tumor development, however, the wild-type allele is inactivatedthrough a somatic mutation, leaving the cell with no functional MMR geneand resulting in a profound defect in MMR activity. Because a somaticmutation in addition to a germ-line mutation is required to generatedefective MMR in the tumor cells, this mechanism is generally referredto as one involving two hits, analogous to the biallelic inactivation oftumor suppressor genes that initiate other hereditary cancers. In linewith this two-hit mechanism, the non-neoplastic cells of HNPCC patientsgenerally retain near normal levels of MMR activity due to the presenceof the wild-type allele (11-12).

While MMR is a conserved process found in bacteria, yeast and mammaliancells (14-16), its activity has not been confirmed in plants. Whilesequences homologous to MMR genes have been identified in Arabidopsisthaliana, it is not known if they are functional in plants in theprocess of MMR (17-18). There is a need in the art for identification ofthe processes involved in genome stability in plants. There is acontinuing need for methods and techniques for generating geneticdiversity in agriculturally important crops.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for making ahypermutable cell.

It is another object of the invention to provide a homogeneouscomposition of cultured, hypermutable, plant cells.

It is still another object of the invention to provide a hypermutabletransgenic plant.

It is yet another object of the invention to provide a method forgenerating a mutation in a gene of interest in a plant cell.

It is still another object of the invention to provide a method forgenerating a mutation in a gene of interest in a plant.

It is an object of the invention to provide a method for generating ahypermutable plant.

It is another object of the invention to provide a vector forintroducing a dominant negative MMR allele into a plant.

It is even another object of the invention to provide an isolated andpurified polynucleotide encoding a plant MutL homolog.

It is another object of the invention to provide an isolated andpurified protein which is a plant MutL homolog.

It is an object of the invention to provide a method for determining thepresence of a mismatch repair (MMR) defect in a plant or a plant cell.

These and other objects of the invention are provided by one or more ofthe following embodiments. In one embodiment of the invention a methodfor making a hypermutable cell is provided. A polynucleotide comprisinga dominant negative allele of a mismatch repair gene is introduced intoa plant cell, whereby the cell becomes hypermutable.

In another aspect of the invention a homogeneous composition ofcultured, hypermutable, plant cells is provided. The plant cellscomprise a dominant negative allele of a mismatch repair gene.

Another aspect of the invention is a hypermutable transgenic plant. Atleast 50% of the cells of the plant comprise a dominant negative alleleof a mismatch repair gene.

According to another aspect of the invention a method is provided forgenerating a mutation in a gene of interest in a plant cell. Ahypermutable plant cell comprising the gene of interest and a dominantnegative allele of a mismatch repair gene is grown. The cell is testedto determine whether the gene of interest harbors a newly acquiredmutation.

Another embodiment of the invention is a method for generating amutation in a gene of interest in a plant. A plant comprising the geneof interest and a polynucleotide encoding a dominant negative allele ofa mismatch repair gene is grown. The plant is tested to determinewhether the gene of interest harbors a newly acquired mutation.

According to another aspect of the invention a method is provided forgenerating a hypermutable plant. Endogenous mismatch repair (MMR)activity of a plant is inhibited. The plant becomes hypermutable as aresult of the inhibition.

Another aspect of the invention is a vector for introducing a dominantnegative MMR allele into a plant. The vector comprises a dominantnegative MMR allele under the transcriptional control of a promoterwhich is functional in a plant.

Still another aspect of the invention provides an isolated and purifiedpolynucleotide encoding Arabidopsis thaliana PMS2 as shown in SEQ ID NO:12.

Another aspect of the invention provides an isolated and purifiedpolynucleotide encoding Arabidopsis PMS134 as shown in SEQ ID NO: 14.

According to another embodiment of the invention an isolated andpurified protein which is Arabidopsis PMS2 is provided. It has the aminoacid sequence as shown in SEQ ID NO: 12.

Another embodiment of the invention is an isolated and purified proteinwhich is Arabidopsis PMS134. It has the amino acid sequence as shown inSEQ ID NO: 14.

Still another aspect of the invention provides a method for determiningthe presence of a mismatch repair (MMR) defect in a plant or a plantcell. At least two microsatellite markers in test cells or a test plantare compared to the at least two microsatellite markers in cells of anormal plant. The test plant or plant cells are identified as having amismatch repair defect if at least two microsatellite markers are foundto be rearranged relative to the cells of the normal plant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-C. Alignment of the Arabidopsis thaliana and human PMS2 cDNAs(SEQ ID NOS: 4 and 3, respectively). Similarity is 48.1%; identity is48.1%. Black boxes show identical nucleotides.

FIG. 2. Alignment of the Arabidopsis thaliana and human PMS2 proteins(SEQ ID NO: 12 and 11, respectively). Similarity is 41.5%; identity is31.1%. Black boxes show identical residues.

FIG. 3. Alignment of the Arabidopsis thaliana MLH1 homolog and the humanPMS2 proteins (SEQ ID NO: 9 and 11, respectively). Similarity is 30%;identity is 18.4%. Black boxes show identical residues.

FIG. 4. Alignment of the Arabidopsis thaliana PMS1 homolog and the humanPMS2 proteins (SEQ ID NO: 10 and 11, respectively). Similarity is 24.4%;identity is 15%. Black boxes show identical residues.

FIG. 5. Phylogenetic tree of Arabidopsis thaliana MutL homologs and thehuman PMS2 protein.

FIG. 6. Alignment of the Arabidopsis thaliana PMS134 and the humanPMS134 cDNA (SEQ ID NO: 6 and 5, respectively). Similarity is 53.2%;identity is 53.2%. Black boxes show identical nucleotides.

FIG. 7. Alignment of the Arabidopsis thaliana PMS134 and the humanPMS134 polypeptides (SEQ ID NO: 14 and 13, respectively). Similarity is65.1%; identity is 50.7%. Black boxes show identical residues.

FIGS. 8 A and B. Western blot analysis of bacteria expressing thehPMS134 (FIG. 8A) or the Arabidopsis thaliana PMS134 (FIG. 8B)polypeptides.

FIG. 9. Expression of plant dominant negative MMR genes produceshypermutability in bacteria, demonstrating the functionality of plantMMR proteins. Briefly, bacteria containing the empty vector or the TACATPMS134 expression vector were grown and elated on kanamycin-containingLbagar plates. The host bacteria are susceptible to KAN bactericidalactivity. Bacterial cultures expressing the hPMS134 gene resulted ingenetic alteration of the bacterial host and the generation of clonesthat are KAN resistant.

FIG. 10. Schematic diagram of a plant dominant-negative MMR expressionvector. Ag7 T. and NOS T.=gene 7 and Nopaline Synthase poly(A) signals,respectively. NOS Prom and CaMV Prom=Nopaline Synthase and CauliflowerMosaic Virus promoters, respectively. L and R=left and right T=DNAborder repeats, respectively. Arrows indicate direction oftranscription.

FIG. 11. Transgenic plants containing the PMS134-KAN vector express thedominant negative hPMS134 gene. Message analysis for T1 plants showssteady state expression of dominant negative MMR genes in PMS134-Kanplants (A lanes) while none is observed in control plants (B lanes).Tubulin was used as an internal control to monitor sample loading andintegrity.

FIG. 12. Microsatellite instability in plants expressing dominantnegative MMR hPMS134 gene.

FIG. 13. MMR defective plants produce new phenotypes. Plants withdecreased MMR produce offspring with two shoot apical meristems (SAM) incontrast to control plants exhibiting a single SAM. Seeds from the MMRdefective plant have been obtained and offspring have the same“double-meristem” trait.

DETAILED DESCRIPTION OF THE INVENTION

It is a discovery of the present inventors that plant cells havefunctional mismatch repair (MMR) systems which function similarly tomammalian MMR. Moreover, dominant negative alleles can be made and usedto generate variability in plants and plant cells, as in mammaliancells. Other means of interfering with normal MMR activity can also beused as described in detail below. Dominant negative alleles of mismatchrepair genes, when introduced into cells or plants, increase the rate ofspontaneous or induced mutations by reducing the effectiveness of DNArepair and thereby render the cells or whole organism hypermutable.Hypermutable plant cells or plants can be utilized to develop newmutations in a gene of interest.

The process of mismatch repair, also called mismatch proofreading, iscarried out by protein complexes in cells ranging from bacteria tomammalian cells (9, 14-16). A mismatch repair (MMR) gene is a gene thatencodes one of the proteins of a mismatch repair complex. Although notwanting to be bound by any particular theory or mechanism of action, amismatch repair complex is believed to detect distortions of a DNA helixresulting from non-complementary pairing of nucleotide bases. Thenon-complementary base on the newer DNA strand is excised, and theexcised base is replaced with the appropriate base, which iscomplementary to the older DNA strand. In this way, cells eliminate manymutations which occur as a result of mistakes in DNA replication.

For purposes of example, this application discloses use of dominantnegative alleles of MMR genes as a method for blocking or inhibiting MMRactivity in plants. (Blocking or inhibiting are used synonymouslyherein, and denote any significant level of inhibition. They do notconnote complete inhibition, although the terms include that possibilitywithin their ambit.) However, any molecular method known by thoseskilled in the art to block MMR gene expression and/or function can beused, including but not limited to gene knockout (19), antisensetechnology (20), double stranded RNA interference (21), and polypeptideinhibitors (22).

Dominant negative alleles cause a mismatch repair defective phenotypeeven in the presence of a wild-type allele in the same cell. An exampleof a dominant negative allele of a mismatch repair gene is the humangene hPMS2-134, which carries a truncation mutation at codon 134 (13,U.S. Pat. No. 6,146,894). The mutation causes the product of this geneto prematurely terminate at the position of the 134th amino acid,resulting in a shortened polypeptide containing the N-terminal 133 aminoacids. Such a mutation causes an increase in the rate of mutations whichaccumulate in cells after DNA replication. Expression of a dominantnegative allele of a mismatch repair gene results in impairment ofmismatch repair activity, even in the presence of the wild-type allele.Any allele which produces such effect can be used in this invention.

Dominant negative alleles of a mismatch repair gene can be obtained fromthe cells of humans, animals, yeast, bacteria, plants or other organismsas described by Nicolaides et. al. (23) and Hori et. al. (24).Alternatively such alleles can be made from wild-type alleles, typicallyby inserting a premature stop codon or other mutation which leads to aprotein product which is able to complex with other members of the MMRcomplex but which is not functional. Such alleles can be identified byscreening cells for defective mismatch repair activity. The cells may bemutagenized or not. Cells from plants exposed to chemical mutagens orradiation, e.g., can be screened for defective mismatch repair. GenomicDNA, a plasmid containing cDNA, or mRNA from any cell encoding amismatch repair protein can be analyzed for variations from the wildtype sequence. Dominant negative alleles of a mismatch repair gene canalso be created artificially, for example, by producing variants of thehPMS2-134 allele or other mismatch repair genes (13, U.S. Pat. No.6,146,894). Other truncated alleles of PMS2 or other MMR genes can bemade. Such alleles are expected to behave similarly to hPMS2-134. An ofvarious forms of site-directed mutagenesis can be used. The suitabilityof such alleles, whether natural or artificial, for use in generatinghypermutable cells or plants can be evaluated by testing the mismatchrepair activity caused by the allele in the presence of one or morewild-type alleles, to determine if the allele is dominant negative.

A cell or a plant into which a dominant negative allele of a mismatchrepair gene has been introduced will become hypermutable. This meansthat the mutation rate (spontaneous or induced) of such cells or plantsis elevated compared to cells or plants without such alleles. The degreeof elevation of the mutation rate can be at least 2-fold, 5-fold,10-fold, 20-fold, 50-fold, 100-fold, 200-fold, 500-fold, or 1000-foldthat of the normal cell or plant.

According to one aspect of the invention, a polynucleotide encoding adominant negative form of a mismatch repair protein is introduced into acell or a transgenic plant. The gene can be any dominant negative alleleencoding a protein which is part of a mismatch repair complex, forexample, mutS or mutL homologs of the bacterial, yeast, fungal, insect,plant, or mammalian genes. The dominant negative allele can be naturallyoccurring or made in the laboratory. The polynucleotide can be in theform of genomic DNA, cDNA, RNA, or a chemically synthesizedpolynucleotide. The polynucleotide can be introduced into the cell bytransfection.

Transfection is any process whereby a polynucleotide is introduced intoa cell. The process of transfection can be carried out in a livingplant, e.g., using a binary vector for gene transmission, or it can becarried out in vitro, e.g., using a suspension of one or more isolatedcells in culture. The cell can be any type of plant cell.

In general, transfection can be carried out using a suspension of cells,or a single cell, but other methods can also be used as long as asufficient fraction of the treated cells incorporates the polynucleotideto allow transfected cells to be readily isolated. The protein productof the polynucleotide may be transiently or stably expressed in thecell. Techniques for transfection are well known in the art of plantcell science. Available techniques for introducing polynucleotidesinclude but are not limited to electroporation, transduction,Agrobacterium-mediated gene transfer, cell fusion, the use of calciumchloride, and packaging of the polynucleotide together with lipid forfusion with the cells of interest. Once a cell has been transfected withthe mismatch repair gene, the cell can, e.g., be grown and reproduced inculture. If the transfection is stable, such that the gene is expressedat a consistent level for many cell generations, then a cell lineresults. Alternatively, a dominant negative MMR protein can be directlyintroduced by microinjection into a cell in order to inhibit MMRactivity of the cell.

Root explants are incubated in 0.5 ug/ml of 2-4-dochlorophenoxy-aceticacid (2-4D) plus N6-Benzyl-Adenine in growth medium. After 4 weeks,suspension cells are isolated and digested with hemicellulase forprotoplast preparation and transfection. Such isolated cells aretypically cultured in the absence of other types of cells. Cellsselected for the introduction of a dominant negative allele of amismatch repair gene may be derived from a multicellular plant in theform of a primary cell culture or an immortalized cell line, or may bederived from suspensions of single-celled plants.

A polynucleotide encoding a dominant negative form of a mismatch repairprotein can be introduced into the genome of a plant to form atransgenic plant. The plant can be any species for which suitabletechniques are available to produce transgenic plants. For example,transgenic plants can be prepared from domestic agricultural crops, e.g.corn, wheat, soybean, rice, sorghum, barley, etc.; from plants used forthe production of recombinant proteins, e.g., tobacco leaf; orexperimental plants for research or product testing, e.g., Arabidopsis,pea, etc. The introduced polynucleotide may encode a protein native tothe species or native to another species, whether plant, animal,bacterial, or fungal, for example.

Any method for making transgenic plants known in the art can be used.According to one process of producing a transgenic plant, thepolynucleotide is transfected into the plant seedling The seed isgerminated and develops into a mature plant in which the polynucleotideis incorporated and expressed. An alternative method for producingtransgenic plants involves introducing the polynucleotide into thegrowing or mature plant by injection, electroporation,Agrobacterium-mediated transfer or transfection. With this method, ifthe polynucleotide is not incorporated into germline cells, the genewill not be passed on to the progeny. Therefore, a transgenic plantproduced by this method will be useful to produce products from thatindividual plant.

To identify whether a gene was inserted into the germline, seedlingsderived from such plants can be screened for the transgene. Geneticmodification of a growing or mature plant is useful for evaluating theexpression of hypermutable constructs and for evaluating effects onaltering endogenous mismatch repair. Once transgenic plants areproduced, they can be grown to produce and maintain a crop of transgenicplants.

Once a transfected cell line or a crop of transgenic plants has beenproduced, it can be used to generate new mutations in one or moregene(s) of interest. A gene of interest can be any gene naturallypossessed by the cell line or transgenic plant or introduced into thecell line or transgenic plant. An advantage of using MMR-defective cellsor plants to induce mutations is that the cell or plant need not beexposed to mutagenic chemicals or radiation, which may have secondaryharmful effects, both on the object of the exposure and on the workers.

Mutations can be detected by analyzing the genotype of the cells orplants, for example by examining the sequence of genomic DNA, cDNA,messenger RNA, or amino acids associated with the gene of interest.Mutations can also be detected by testing a phenotype caused by thegene. A mutant phenotype can be detected, e.g., by identifyingalterations in electrophoretic mobility, spectroscopic properties, orother physical or structural characteristics of a protein encoded by amutant gene. One can also screen for altered function of the protein insitu, in isolated form, or in model systems. One can screen foralteration of any property of the cell or plant associated with thefunction of the gene of interest or its protein product. Finally, onecan screen for macroscopic phenotypes such as but not limited to color,height, or the ability to grow in drought, high-salt, cold, hot, acidic,basic, pest-infested, or high ethylene environments.

The above disclosure generally describes the present invention. A morecomplete understanding can be obtained by reference to the followingspecific examples that will be provided herein for purposes ofillustration only, and are not intended to limit the scope of theinvention.

EXAMPLE 1 Isolation of Plant Mismatch Repair Genes

The ability to increase the hypermutability of host genomes has manycommercial and medical applications. The generation of hypermutableplants such as those used in agriculture for livestock feed and humanconsumption are just one example of many types of applications that canbe generated by creating hypermutable organisms. For instance, thecreation of crops that are less susceptible to pests or soil pH wouldgreatly increase yield of certain agricultural crops. In addition togreater production of goods, improved crops could increase the abilityto grow many generations of crops on the same fields (25-27). Moreover,the ability to affect certain growth traits such as naturalpest-resistance, drought-resistance, frost-resistance, increasedproduction, or altered stalk size has many benefits for the productionof agricultural products. Recently, it has been demonstrated that genesaffecting the biologic activity of the plant growth hormone gibberellinresults in crops with shorter stalk length that produce similar amountsof grain yields, however, the fact that the stalks are shorter makesthese plants less susceptible to high winds and crop damage (28). Theuse of hypermutable crops could allow for the selection of shorterplants across many species such as corn, rice, etc, without having tofirst identify a gene to alter its activity. Another application ofhypermutable agricultural products is the generation of crops withenhanced levels of vitamins and nutrients. One can select for enhancedvitamin production in seedlings of MMR defective plants. Recently, ithas been demonstrated that altering a gene(s) within a vitaminbiosynthetic pathway can result in the production of elevated levels ofvitamin E (27,29).

Applications of hypermutable plants include use as crops foragricultural production, increased medicinal entities within plantextracts, chemicals and resins for industrial use, and their use asdetoxifying organisms for environmental applications as described(25,26,29).

MutS and mutL homologs can be isolated from plant species librariesusing degenerate RT-PCR, and standard Southern hybridization techniquesas previously described (3,23,30). These may serve as reagents forproducing MMR defective plant hosts. This process employs methods knownby those skilled in the art of gene cloning.

One such approach is the use of degenerate PCR to clone MutS homologsfollowing the methods used by Leach et. al. to clone the human MSH2 (3).Additional degenerate oligonucleotides can be generated and used againstconserved domains of bacterial, yeast, and human MutS homologs. Variousplant species cDNAs (obtainable from various commercial sources) can beamplified for MutS gene homologs by polymerase chain reaction (PCR).Products are cloned into T-tailed vectors (In Vitrogen) and analyzed byrestriction endonuclease digestion. Clones with expected DNA fragmentinserts are sequenced using M13 forward and reverse primers located onthe vector backbone flanking the cloning site. Fragments containing MMRgene homologs are then used as probes to screen commercially availablecDNA libraries from the appropriate species. cDNA contigs are generatedto create a cDNA containing the sequence information for the full lengthMMR gene and its encoded polypeptide. One such example of cloning aplant MMR gene is provided below.

In order to clone mutL homologs, degenerate primers were synthesized tothe conserved domains of the mutL gene family by aligning E. coli,yeast, mouse, and human mutL genes. These primers are directed to thepolynucleotide sequences centered at nt 150 to 350 of the publishedhuman PMS2 cDNA (SEQ ID NO: 3). Degenerate PCR was carried out using RNAfrom Arabidopsis thaliana (AT) that was isolated using the RNeasy kitfollowing the manufacturer's protocol (Qiagen). RNAs were reversetranscribed (RT) using Superscriptf1 (Life Technologies) following themanufacturer's protocol. After RT, cDNAs were PCR amplified usingdegenerate primers in buffers described by Nicolaides et. al. 1995(23,30), and reactions were carried out at 95° C. for 30 sec for 1 cyclefollowed by 94° C. for 30 sec, 45° C. for 60 sec, and 72° C. for 60 secfor 20 cycles. PCR reactions were then diluted 1:10 in water andreamplified using the same primers and buffers. The secondary PCRreactions were carried out at 95° C. for 30 sec for 1 cycle followed by94° C. for 30 sec, 52° C. for 90 sec, and 72° C. for 90 sec for 35cycles. Reactions were analyzed by agarose gel electrophoresis. Productsof the expected molecular weight were excised and cloned into T-tailedvectors (InVitrogen). Recombinant clones were sequenced and blastedagainst the public databases. The homolog was found to have homology tothe mutL family of genes. Blast search analysis of GenBank found thisgene to be part of a “putative” mismatch repair gene identified from theArabidopsis genome project that has never been reported to betranscribed or capable of producing a message. In order to clone thefull length, an Arabidopsis cDNA library was screened by PCR as well ascDNA from AT plants using 5′ primers corresponding to the initiationcodon (SEQ ID NO: 1: 5′-atg caa gga gat tct tc-3′) and the terminationcodon (SEQ ID NO: 2: 5′-tca tgc caa tga gat ggt tgc-3′) using buffersand conditions listed above. Amplifications were carried out at 95° C.for 30 sec for 1 cycle followed by 94° C. for 30 sec, 58° C. for 2 mm,and 72° C. for 3 mm for 35 cycles. Products were analyzed by gelelectrophoresis. Products of the expected molecular weights weresubcloned into T-tail vectors and sequenced using primers from thecloning vector or using internal primers. FIG. 1 shows the alignment ofone Arabidopsis homolog, referred to as ATPMS2 (SEQ ID NO: 4), to thehuman PMS2 cDNA (SEQ ID NO:3) (FIG. 1) and the hPMS2 protein (FIG. 2;SEQ ID NO: 11). This gene was found to be homologous (4800 identity) tothe human PMS2 (SEQ ID NO:3) cDNA and its encoded polypeptide (31%identity) (FIG. 2). Other homologs to the A TPMS2 were also identifiedfrom blast searching sequence databases. One mutL homolog is closelyrelated to the MLH1 mammalian homolog and is referred to as ATMLH1(shown in FIG. 3) and another is closely related to the mammalian PMS1polypeptide referred to as ATPMS1 (shown in FIG. 4). A phylogenetic treeis shown in FIG. 5 showing the homology of the mutL homologs to thehuman PMS2 gene.

Degenerate primers can be used for isolating MMR genes from other plantspecies in a similar fashion.

EXAMPLE 2 Generation of Dominant Negative Alleles of Plant MismatchRepair Genes

To demonstrate that putative plant MMR proteins are truly involved inMMR biochemical process, cDNAs are cloned into constitutive (31,32) orinducible (33) bacterial expression vectors for functional studies.Various deletion mutants are generated to produce dominant negative MMRgenes. Dominant negative alleles that are identified in the bacterialsystem are then useful for plant studies. Dominant negative MMR genesare prepared by over-expression of full-length MMR genes or by deletionanalysis using standard protocols used by those skilled in the art ofmolecular biology. One such dominant MMR gene mutant was created bygenerating a construct with similar domains to that of the humandominant negative PMS2 gene (referred to as PMS134) (13, U.S. Pat. No.6,146,894). To generate this vector, the ATPMS2 (SEQ ID NO: 4) and hPMS2cDNA (SEQ ID NO: 3) sequences were aligned and the conserved domain wasisolated. FIG. 6 shows a sequence alignment between the human and ATPMS134 cDNAs where a 5200 identity is found between the two sequences.At the protein level these domains have a 51% identity (FIG. 7).Dominant negative hPMS134 and ATPMS134 genes were made by PCR andsubcloned into bacterial expression vectors. The ATPMS134 was generatedby PCR from the cloned cDNA using a sense primer (SEQ ID NO:1)corresponding to the N-terminus and an antisense primer (SEQ ID NO: 15)5′gtcgacttatcacttgtcatcgtcgtccttgtagtc gagcgtagcaactggctc-3′ centered atnt 434 of the ATPMS2 cDNA (SEQ ID NO:4). This primer also contains aflag epitope that will allow protein detection followed by twotermination codons. PCR products of the expected molecular weight weregel purified and cloned into T-tail vectors. Recombinant clones weresequenced to ensure authentic sequences. Inserts were then cloned intothe inducible pTAC expression vector, which also contains the Ampicillinresistance gene as a selectable marker. The human PMS134 allele was alsocloned into the pTAC expression vector as a positive control.Electrocompetent DH5alpha and DH10b bacterial cells (Life Technologies)were electroporated with empty vector, and the loaded vectorspTACATPMS134 and pTAChPMS134, using an electroporator (BioRAd) followingthe manufacturer's protocol. Bacterial cultures were then plated on toLB agar plates containing 100 μg/ml ampicillin and grown at 37° C. for14 hours. Ten recombinant clones were then isolated and grown in 5 mlsof LB broth containing 50 μg/ml ampicillin plus 50 μM IPTG for 18 hr at37° C. One hundred microliters were then collected, spun down, anddirectly lysed in 2.times. SDS buffer for western blot analysis. Forwestern analysis, equal number of cells were lysed directly in 2×SDSbuffer (60 mM Iris, pH 6.8, 2% SDS, boo glycerol, 0.1 M2-mercaptoethanol, 0.001% bromophenol blue) and boiled for 5 minutes.Lysate proteins are separated by electrophoresis on 4-12% NuPAGE gels(Novex). Gels are electroblotted onto Immobilon-P (Millipore) in 48 mMIris base, 40 mM glycine, 0.037500 SDS, 20% methanol and blockedovernight at 4° C. in Tris-buffered saline plus 0.05% Tween-20 and 5%condensed milk. Filters are probed with a polyclonal antibody generatedagainst MMR polypeptide sequence or a fused tag (e.g. FLAG, HIS, etc.)and a horseradish peroxidase conjugated secondary antibody, usingchemiluminescence for detection (Pierce). FIG. 8 shows a western blot ofa clone that expresses the human PMS134 protein (FIG. 8A, lane 2) usinga human PMS2-specific antibody (directed to residues 2-20) of thehPMS134 sequence (see FIG. 1, and SEQ ID NO:6) or the Arabidopsis PMS134protein (FIG. 8B, lane 2) using an anti-FLAG antibody directed to thefusion residues at the C-terminus of the protein. Cells expressing emptyvector had no detectable expression.

Bacterial clones expressing the hPMS134, ATPMS134 or the empty vectorwere grown in liquid culture for 24 hr at 37° C. in the presence of 50μg/ml ampicillin plus 50 μM IPTG. The next day, cultures were diluted1:10 in medium containing 50 μM IPTG plus ampicillin or ampicillin plus25 μg/ml kanamycin (AK) and cultures were grown for 18 hr at 37° C. Thefollowing day, a 0.1 μl aliquot (2 μl diluted in 1000 μl of LB mediumand used 50 μl for plating) of cells grown in Amp medium were plated onLB-agar plates containing 40 μg/ml of5-bromo-4-chloro-3-indolyl-B-D-galactoside (X-gal) plus 100 μg/mlampicillin (AMP), while a 1 μl aliquot (1 μl diluted in 100 μl of LBmedium and used 100 μl for plating) of cells grown in AK medium wereplated on LB-agar plates containing X-gal and 50 μg/ml kanamycin (KAN).Plates were incubated for 18 hours at 37° C. The results from thesestudies show that cells expressing the hPMS134 or the ATPMS134polypepetides displayed increased mutation rates in the genome of theDH10B bacterial strain which resulted in the production of KAN resistantclones (FIG. 9). Following the mutagenesis protocol described above,bacterial cells expressing the plant ATPMS134 were found to have anincrease in the number of KAN resistant cells (12 clones) in contrast tocells expressing the empty vector, which yielded no KAN resistant clone.These data demonstrate that dominant negative ATPMS134 MMR genes areuseful for creating hypermutable organisms that can generatephenotypically diverse offspring when put under selective conditions.Moreover, these data demonstrate that plants also use the conserved MMRprocess for genomic stability.

Dominant negative plant MMR gene mutants are also analyzed usingmammalian cell systems. In this case, plant MMR gene cDNAs are clonedinto eukaryotic expression vectors as described (13,34) and cellsexpressing dominant negative mutants are analyzed by measuring stabilityof endogenous microsatellite markers and biochemical activity of cellextracts from lines expressing dominant negative MMR gene alleles. Suchmethods are known by those skilled in the art and previously described(13).

EXAMPLE 3 Inhibition of Plant MMR Activity by Dominant Negative MMRAlleles Produces Genetic Hypermutability and Microsatellite Instability

Dominant negative alleles of human and AT MMR genes identified usingbacterial and or mammalian systems can be used for plants. To test thehypothesis that dominant negative MMR gene alleles produce globalhypermutability in plants, the hPMS134 and ATPMS134 cDNAs were expressedin plants. These alleles have been found to work across species wherethe introduction of these genes into MMR proficient bacterial ormammalian cells renders the cells MMR deficient. Assays to carry outthese studies are described below.

Engineering Plant Expression Vectors to Express the PMS134 DominantNegative Alleles

A BamH I fragment containing the hPMS134 cDNA was obtained from thepSG5PMS134 plasmid (ref 13) and cloned into the corresponding sites ofthe pEF1/SP1-V5 vector (InVitrogen). The resulting vector(pEF-PMS134-sense) was then digested with Pme I to release a blunted DNAfragment containing the PMS134 cDNA. This fragment was then subclonedinto the blunt Sma I and EcoICR I sites of the pGPTV-KAN binary plantexpression vector (American Type Culture Collection). One clone, namedpCMV-hPMS134-Kan (see FIG. 10), was sequenced to confirm that the vectorcontained authentic promoter and gene insert sequences. A schematicdiagram of the pCMV-hPMS134-Kan vector is shown in FIG. 10.

Generation of hPMS134-Expressing Arabidopsis Thaliana Transgenic Plants

Agrobacterium tumefaciens cells (agrobacteria) are used to shuttle genesinto plants. To generate PMS134-expressing Arabidopsis thaliana (A.thaliana) plants, Agrobacterium tumefaciens cells (strain GV3101) wereelectroporated with pCMV-hPMS134-Kan or the pBI-121 (BRL) control binaryvector. The pBI-121 control contains the CaMV promoter driving theexpression of the β-glucuronidase cDNA (GUS) and serves as a control.Both vectors carry the neomycin phosphotransferase (NPTII) gene thatallows selection of agrobacteria and plants that contain the expressionvector. One-month old A. thaliana (ecotype Columbia) plants wereinfected by immersion in a solution containing 5% sucrose, 0.05% silwet,and binary vector-transformed agrobacteria cells for 10 seconds. Theseplants were then grown at 25° C. under a 16 hour day and 8 hour darkphotoperiod. After 4 weeks, seeds (referred to as T1) were harvested anddried for 5 days at 4° C. Thirty thousands seeds from tenCMV-hPMS134-Kan-transformed plants and five thousand seeds from twopBI-121-transformed plants were sown in solid Murashige and Skoog (MS)media plates in the presence of 50 μg/ml of kanamycin (KAN). Threehundred plants were found to be KAN resistant and represented PMS134expressing plants. These plants along with 300 control plants were grownin MS media for two weeks and then transferred to soil. Plants weregrown for an additional four weeks under standard conditions at whichtime T2 seeds were harvested.

To confirm the integration and stability of the PMS134 gene in the plantgenome, gene segregation and PCR analyses were conducted. Commonly,three out of four T1 plants transformed by agrobacteria technology areexpected to carry the vector inserted within a single locus and aretherefore considered heterozygous for the integrated gene. Approximately75% of the seeds (T2) generated from most of the T1 plants were foundKAN-resistant and this in accordance with the expected 1:2:1 ratio ofnull (no hPMS134 containing plants), heterozygous, and homozygousplants, respectively, in self-pollinating conditions. To confirm thatthese plants contained the hPMS134 expression vector, genomic DNA wasisolated from leaves of T1 plants using the DNAzol-mediated techniquefollowing the manufacturer's suggestion (BRL-Life Technologies). Onenanogram of genomic DNA was analyzed by polymerase chain reaction (PCR)to confirm the presence of the hPMS134 gene. PCR was carried out for 25cycles with the following parameters: 95° C. for 30 seconds; 55° C. for1 minute; and 72° C. for 2 minutes using hPMS134-specific sense (SEQ IDNO: 7: 5′-tct aga cat gga gcg agc tga gag ctc-3′) and antisense (SEQ IDNO: 8: 5′-tct aga agt tcc aac ctt cgc cga tgc-3′) primers. Positivereactions were observed in DNA from pCMV-hPMS134-Kan-transformed plantsand not from pBI-121-transformed plants, thus confirming the integrationof this vector.

In order to assess the expression of hPMS134 in T1 plants, leaf tissuewas collected from T1 plants, homogenized in liquid nitrogen using glasspestles, and suspended in RLT lysing buffer (Qiagen, RNeasy plant RNAextraction kit). Five micrograms of total RNA was purified according tothe manufacturer's suggested protocol and then loaded onto a 1.2%agarose gel (1× MOPS buffer, 3% formaldehyde), size-fractionated byelectrophoresis, and transferred onto N-Hybond+ membrane (Amersham).Each membrane was incubated at 55° C. in 10 ml of hybridization solution(North2South labeling kit, Pierce) containing 100 ng of PMS134, tubulin,or KAN cDNA probes, which were generated by PCR amplification, accordingto the manufacturer's directions. Membranes were washed three times in2×SSC, 0.1% SDS at 55° C., and three times in 2×SSC at ambienttemperature. Detection was carried out using enhanced chemiluminescence(ECL). Expression was also carried out by reverse trascriptase PCR asdescribed above using polyA isolated mRNA that was isolated over a oligodT column (Qiagen). A representative example of these studies are shownin FIG. 11. Here hPMS134 expression was detected in three out of tenanalyzed pCMV-hPMS134-Kan transgenic lines, while no signal was found inthe ten pBI-121 transformed plants analyzed. Immunoblot using wholelysates is used to confirm protein expression. Collectively thesestudies demonstrate the generation of hPMS134 expressing transgenic A.thaliana plants.

Molecular Characterization of PMS134-Expressing Plants

MMR is a process that is involved in correcting point mutations and“slippage” mutations within repetitive mono-, di-, and tri-nucleotide(microsatellite) repeats that occur throughout the genome of an organismafter cellular replication. This process is very similar to a computerspell check function. The inactivation of MMR has been shown to resultin global genomic hypermutation whereby cells with defective MMRexperience over a one thousand-fold increase in point mutations andmicrosatellite instability (MI) (mutations within repetitive sequences)throughout their genomes per division. (35). MMR deficiency is the onlyknown process capable of producing MI and has been used as a marker todetect cells with MMR dysfunction (36). Microsatellites serve asmolecular tools to measure the inactivation of MMR that occurs by thedefective MMR due to but not limited to expression of dominant negativeMMR genes, double stranded RNA interference vectors, or inhibition byantisense nucleotides, or by gene knockout. In A. thaliana, a series ofpoly-A (A)n, (CA)n and (GA)n sequences were identified from genomesearches using EMBL and GenBank databases. To demonstrate that hPMS134expression could produce MI in A. thaliana, we analyzed microsatellitesin T1 plants by PCR analyses. Initially we monitored threemicrosatellites, ATHACS, Nga280, and Nga128 with published primers thathave been previously used to map the Arabidopsis genome (37). Briefly,DNA was extracted from A. thaliana leaves as described above. 10 ngs ofplant genomic DNA was used as template for PCR amplification using thefollowing amplification conditions: 94° C. for 15 sec, 55° C. for 15 secand 72° C. for 30 seconds. PCR products were analyzed on 5% Metaphoragarose (BioWhittaker Molecular Applications) and ethidium bromidestaining. In one transgenic pCMV-PMS134-Kan line, we detected a doubleproduct, likely representing a new allele of the polymorphic nga280locus (FIG. 12). These data demonstrate the ability to produce MMRdeficiency and MI in plants expressing the hPMS134 dominant negativeallele and provide a molecular tool for screening MMR-defective plants.

Biochemical assays for mismatch repair. MMR activity in nuclear extractsis performed as described, using 24 fmol of substrate as described (13).Complementation assays are done by adding ˜100 ng of purified MutL orMutS components to 100 μg of nuclear extract, adjusting the final KClconcentration to 100 mM. The substrates used in these experiments willcontain a strand break 181 nucleotides 5′ or 125 nucleotides 3′ to themismatch.

EXAMPLE 4 Inactivation of MMR Leads to Plants with New Phenotypes

We demonstrated the ability of the defective MMR to produce molecularchanges within plants. The objective of this section is to demonstratethe ability to generate MMR defective plants with macroscopic outputtraits. One way to measure for plants with new phenotypes is to growplants under toxic conditions, such as but not limited to high levels oftoxic ions, pest-infection, drought conditions, or extreme temperaturesto identify a minority of plants with new output traits, i.e.,resistance. Another way to score for plants with new phenotypes isthrough physical differences of MMR defective plants grown in standardconditions. An example of MMR-defective plants with new phenotypesinclude the generation of plants with double shoot apical meristems(FIG. 13) as well as plants with altered chlorophyll productionrendering plants albino (data not shown). In FIG. 13, we show a typicalwild type plant (left, labeled normal) and a plant produced from the MMRdefective group (right, labeled MMR deficient). The double-meristemtrait was not observed in greater than 500 normal plants. Thedouble-meristem trait does not appear to be due to transgene integrationsince segregation analysis reveals the ability to generatedouble-meristem plants in the absence of transgene positive plants whileMMR proficient control plants with other transgene vectors (pBI-121) didnot produce this phenotype (data not shown). These data suggest thatdefective MMR produced a mutation or mutations within the plant genomethat altered the normal biochemical function of the host to produce anew output trait.

These data demonstrate the ability to create plant subtypes with newgenetic and phenotypic traits by blocking the endogenous MMR process ofthe plant cell or whole organism.

EXAMPLE 5 Inhibition of Plant MMR Activity Using Molecular Approaches

This application teaches of the use of inhibiting MMR activity in aplant to produce genetically altered offspring with new phenotypes.

The inhibition of MMR activity in a host organism can be achieved byintroducing a dominant negative allele as shown in FIG. 11 and 12. Otherways to suppress MMR activity of a host is by: knocking out alleles of aMMR protein through homologous recombination (38); blocking MMR proteindimerization with other subunits (which is required for activity) by theintroduction of polypeptides into the host via transfection methods;knocking out the expression of a MMR gene using anti-senseoligonucleotides (20), and/or the use of double stranded RNAinterference genes (21).

MMR Gene Knockouts

Data shown in EXAMPLE 1 demonstrate that plants contain MMR genehomologs that can be genetically engineered to produce alteredbiochemical activities. Data presented in EXAMPLES 3 and 4 demonstratethat defective MMR in plants can produce hypermutable parental plantsand offspring. Together, these data demonstrate that inhibitingendogenous MMR genes by targeting vectors of the particular MMR gene canlead to hypermutability of a plant host that generate offspring withaltered genetic loci and/or new phenotypes as described in EXAMPLES 3,4, and 5. Hypermutable seedlings can also be produced with “knocked out”MMR genes using methods known by those skilled in the art. These can beused to produce genetically diverse offspring for commercial and medicalapplications (38). Cells will be confirmed to have lost the expressionof the MMR gene using standard northern techniques and determined to beMMR defective using microsatellite analysis as described in EXAMPLE 3.

Blocking Polypeptides

MMR subunits (MutS and MutL proteins) interact to form active MMRcomplexes. Peptides are able to specifically inhibit the binding of twoproteins by competitive inhibition. Isolation of plant MMR genes allowsfor the elucidation of primary amino acid structures as described inEXAMPLE 1. Peptides containing some but not all of the domains can besynthesized from domains of the particular MMR gene and introduced intohost plants using methods known by those skilled in the art (22). Liketruncated PMS134, such peptides will compete with functional full lengthproteins for binding and form enzymatically inactive MMR complexes. Thedata indicate that the domains which are C-terminal to the 134 positionin human PMS2 are dispensible for binding and necessary for enzymaticactivity. As shown herein, a similar domain structure is also found inplant PMS2. Seedlings exhibiting hypermutability will be useful toproduce genetically diverse offspring for commercial and medicalapplications.

RNA blockade and Double Stranded Interference

MMR subunits (MutS and MutL proteins) interact to form active MMRcomplexes. Peptides are able to specifically inhibit the binding of twoproteins by competitive inhibition. Antisense oligonucleotides aresynthesized against the cDNA sequence of plant MMR homologs identifiedin EXAMPLE 1 (20). Antisense molecules are then introduced into hostplants using methods described in EXAMPLE 2 or through the bathing ofseedlings or plantlets. Seedlings exhibiting hypermutability will beuseful to produce genetically diverse offspring for commercial andmedical applications.

Double stranded interference vectors are also useful for blockingexpression/function of a plant MMR gene. The plant gene is expressed inboth sense and antisense orientations from a transfection vector and theendogenous gene expression is suppressed by endogenous silencingprocesses (21).

Discussion

Plants contain MMR genes that code for MMR functional proteins.Expression of dominant negative plant MMR proteins results in anincrease in microsatellite instability and hypermutability in plants.This activity is due to the inhibition of MMR biochemical activity inthese hosts. The data provided within this application demonstrates theblockade of MMR in a plant to produce genetic changes that lead to theproduction of offspring or cells with new output traits. This method isapplicable to generate crop plants with new output traits as well asplant cells exhibiting new biochemicals for commercial use.

REFERENCES CITED (EACH OF WHICH IS EXPRESSLY INCORPORATED HEREIN FOR ALLPURPOSES)

-   1. Liu, B., Parsons, R., Papadopoulos, N., Nicolaides, N. C.,    Lynch, H. T., Watson, P., Jass, J. R., Dunlop, M., Wyllie, A.,    Peltomaki, P., de la Chapelle, A., Hamilton, S. R., Vogelstein, B.,    and Kinzler, K. W. 1996. Analysis of mismatch repair genes in    hereditary non-polyposis colorectal cancer patients. Nat. Med.    2:169-174.-   2. Bronner, C. E., Baker, S. M., Morrison, P. T., Warren, G.,    Smith, L. G., Lescoe, M. K., Kane, M., Earabino, C., Lipford, J.,    Lindblom, A., Tannergard, P., Bollag, R. J., Godwin, A., R.,    Ward, D. C., Nordenskjold, M., Fisbel, R., Kolodner, R., and    Liskay, R. M. 1994. Mutation in the DNA mismatch repair gene    homologue hMLH1 is associated with hereditary non-polyposis colon    cancer. Nature 368:258-261.-   3. Leach, F. S., Nicolaides, N. C, Papadopoulos, N., Liu, B., Jen,    J., Parsons, R., Peltomaki, P., Sistonen, P., Aaltonen, L. A.,    Nystrom-Lahti, M., Guan, X. Y., Zhang, J., Meltzer, P. S., Yu, J.    W., Kao, F. T., Chen, D. J., Cerosaletti, K. M., Foumier, R. E. K.,    Todd, S., Lewis, T., Leach R. J., Naylor, S. L., Weissenbach, J.,    Mecklin, J. P., Jarvinen, J. A., Petersen, G. M., Hamilton, S. R.,    Green, J., Jass, J., Watson, P., Lynch, H. T., Trent, J. M., de la    Chapelle, A., Kinzler, K. W., and Vogelstein, B. 1993. Mutations of    a mutS homolog in hereditary non-polyposis colorectal cancer. Cell    75:1215-1225.-   4. Nicolaides, N. C., Papadopoulos, N., Liu, B., Wei, Y. F.,    Carter, K. C., Ruben, S. M., Rosen, C. A., Haseltine, W. A.,    Fleischmann, R. D., Fraser, C. M., Adams, M. D., Venter, C. J.,    Dunlop, M. G., Hamilton, S. R., Petersen, G. M., de la Chapelle, A.,    Vogelstein, B., and kinzler, K. W. 1994. Mutations of two PMS    homologs in hereditary nonpolyposis colon cancer. Nature 371: 75-80.-   5. Nicolaides, N. C., Palombo, F., Kinzler, K. W., Vogelstein, B.,    and Jiricny, J. 1996. Molecular cloning of the N-terminus of GTBP.    Genomics 31:395-397.-   6. Palombo, F., Gallinari, P., Iaccarino, I., Lettieri, T.,    Hughes, M. A., Truong, O., Hsuan, J. J., and Jiricny, J. 1995. GTBP,    a 160-kilodalton protein essential for mismatch-binding activity in    human cells. Science 268:1912-1914.-   7. Papadopoulos, N., Nicolaides, N. C., Wei, Y. F., Carter, K. C.,    Ruben, S. M., Rosen, C. A., Haseltine, W. A., Fleischmann, R. D.,    Fraser, C. M., Adams, M. D., Venter, C. J., Dunlop, M. G.,    Hamilton, S. R., Petersen, G. M., de la Chapelle, A., Vogelstein,    B., and kinzler, K. W. 1994. Mutation of a mutL homolog is    associated with hereditary colon cancer. Science 263:1625-1629.-   8. Perucho, M. 1996. Cancer of the microsatellite mutator phenotype.    Biol Chem. 377:675-684.-   9. Strand, M., Prolla, T. A., Liskay, R. M., and Petes, T. D.    Destabilization of tracts of simple repetitive DNA in yeast by    mutations affecting DNA mismatch repair. 1993. Nature 365:274-276.-   10. Ma A H, Xia L, Littman S J, Swinler S, Lader G, Polinkovsky A,    Olechnowicz J, Kasturi L, Lutterbaugh J, Modrich P, Veigl M L,    Markowitz S D, Sedwick W D. 2000. Somatic mutation of hPMS2 as a    possible cause of sporadic human colon cancer with microsatellite    instability. Oncogene 19:2249-2256.-   11. Parsons, R., Li, G. M., Longley, M. J., Fang, W. H.,    Papadopolous, N., Jen, J., de la Chapelle, A., Kinzler, K. W.,    Vogelstein, B., and Modrich, P. 1993. Hypermutability and mismatch    repair deficiency in RER⁺ tumor cells. Cell 75:1227-1236.-   12. Li, G. M., and Modrich, P. 1995. Restoration of mismatch repair    to nuclear extracts of H6 colorectal tumor cells by a heterodimer of    human mutL homologs. Proc. Nat. Acad. Sci. USA 92:1950-1954.-   13. Nicolaides N. C. et. al. 1998. A naturally occurring hPMS2    mutation can confer a dominant negative mutator phenotype. Mol Cell    Biol. 18:1635-1641.-   14. Modrich, P. 1994. Mismatch repair, genetic stability, and    cancer. Science 266:1959-1960.-   15. Pang, Q., Prolla, T. A., and Liskay, R. M. 1997. Functional    domains of the Saccharomyces cerevisiae Mlh1p and Pms1p DNA mismatch    repair proteins and their relevance to human hereditary nonpolyposis    colorectal cancer-associated mutations. Mol. Cell. Biol.    17:4465-4473.-   16. Harfe, B. D., and Jinks-Robertson, S. 2000. DNA MISMATCH REPAIR    AND GENETIC INSTABILITY. Annu Rev Genet 34:359-399.-   17. Culligan, K. M, and Hays, J. B. 1997. DNA mismatch repair in    plants. An Arabidopsis thaliana gene that predicts a protein    belonging to the MSH2 subfamily of eukaryotic MutS homologs. Plant    Physiol. 115:833-839.-   18. Jean M, Pelletier J, Hilpert M, Belzile F, and Kunze R. 1999.    mutl Plant Isolation and characterization of ATMLH1, a MutL    homologue from Arabidopsis thaliana. Mol Gen Genet 262:633-642.-   19. de Wind N., Dekker, M., Berns, A., Radman, M., and    Riele, H. T. 1995. Inactivation of the mouse Msh2 gene results in    mismatch repair deficiency, methylation tolerance,    hyperrecombination, and predisposition to cancer. Cell 82:321-300.-   20. Hackett R M, Ho C W, Lin Z, Foote H C, Fray R G,    Grierson D. 2000. Antisense inhibition of the Nr gene restores    normal ripening to the tomato never-ripe mutant, consistent with the    ethylene receptor-inhibition model. Plant Physiol 124:1079-1086.-   21. Chuang C F, Meyerowitz E M. 2000. Specific and heritable genetic    interference by double-stranded RNA in Arabidopsis thaliana. Proc    Natl Acad Sci USA 97:4985-4990.-   22. Takeuchi M, Ueda T, Sato K, Abe H, Nagata T, and Nakano A. 2000.    A dominant negative mutant of sar1 GTPase inhibits protein transport    from the endoplasmic reticulum to the golgi apparatus in tobacco and    arabidopsis cultured cells. Plant J. 23:517-525.-   23. Nicolaides, N. C., Carter, K. C., Shell, B. K., Papadopoulos,    N., Vogelstein, B., and Kinzler, K. W. 1995. Genomic organization of    the human PMS2 gene family. Genomics 30:195-206.-   24. Horii, A., et. al. 1994. Cloning, characterization and    chromosomal assignment of the human genes homologous to yeast PMS1,    a member of mismatch repair genes. Biochem Biophys Res Commun.    204:1257-1264.-   25. Moffat, A. S., 1999. Engineering plants to cope with metals.    Science 285:369-370.-   26. Mazur, B., Krebbers, E., and Tingey, S. 1999. Gene discovery and    product development for grain quality traits. Science. 285:372-375.-   27. Shintani, D., and DellaPenna, D. 1998. Elevating the Vitamin E    content of plants through metabolic engineering. Science.    282:2098-2100.-   28. Peng, J., Richards, D. E., Hartley, N. M., Murphy, G. P., et.    al. 1999. “Green revolution” genes encode for mutant gibberellin    response modulators. Nature. 256-261.-   29. DellaPenna, D. 1999. Nutritional genomics: Manipulating plant    micronutrients to improve human health. Science. 285:375-379.-   30. Nicolaides N. C., Kinzler, K. W., and Vogelstein, B. 1995.    Analysis of the 5′ region of PMS2 reveals heterogenous transcripts    and a novel overlapping gene. Genomics 29:329-334.-   31. Murai, N., Sutton, D. W., Murray, M. G., Slightom, J. L.,    Merlo, D. J., et. al. 1983. Science. 222:476-???.-   32. Koziel, M. G., Adams, T. L., Hazlet, M. A., et. al., 1984. J.    MOL. Appl. Genet. 2:549-560.-   33. Kaulen, H., Schell, J., and Kreuzaler, F. 1986. EMBO J. 5:1-   34. Nicolaides, N. C., Correa, I., Casadevall, C., Travali, S.,    Soprano, K. J., and Calabretta, B. 1992. The Jun family members,    c-JUN and JUND, transactivate the human c-myb promoter via an Ap1    like element. J. Biol. Chem. 267, 19665-19672.-   35. Wheeler J M, Loukola A, Aaltonen L A, Mortensen N J, and Bodmer    W F. 2000. The role of hypermethylation of the hMLH1 promoter region    in HNPCC versus MSI+ sporadic colorectal cancers. J. Med. Genet.    37:588-592.-   36. Hoang J M, Cottu P H, Thuille B, Salmon R J, Thomas G, and    Hamelin R. 1997. BAT-26, an indicator of the replication error    phenotype in colorectal cancers and cell lines. Cancer Res    57:300-303.-   37. Bell, C. J., et. al. Genomics, 19:137-144, 1994.-   38. Gal, S., Pisan, B., Hohn, T., Grimsley, N., and Hohn, B. 1991.    Genomic homologous recombination in planta. EMBO J. 10:1571-1578.

1. A method for generating a mutation in a gene of interest in a plantcell, comprising the steps of: introducing into a plant cell comprisingthe gene of interest a polynucleotide encoding human PMS2-134 protein orArabidopsis PMS2-134 protein, wherein said polynucleotide has atermination codon at codon 134 or after codons 1-147, respectively,which when expressed forms a truncated PMS2 protein, whereby the plantcell becomes hypermutable; growing the hypermutable plant cell; andtesting the cell to determine whether the gene of interest harbors amutation.
 2. The method of claim 1 wherein the step of testing comprisesanalyzing a nucleotide sequence of the gene of interest.
 3. The methodof claim 1 wherein the step of testing comprises analyzing mRNAtranscribed from the gene of interest.
 4. The method of claim 1 whereinthe polynucleotide encodes human PMS2-134.
 5. The method of claim 4wherein the step of testing comprises analyzing a nucleotide sequence ofthe gene of interest.
 6. The method of claim 4 wherein the step oftesting comprises analyzing mRNA transcribed from the gene of interest.7. A method for generating a mutation in a gene of interest in a plant,comprising the steps of: introducing into a plant comprising the gene ofinterest a polynucleotide encoding human PMS2-134 protein or ArabidopsisPMS2-134 protein wherein said polynucleotide has a termination codon atcodon 134 or after codons 1-147, respectively, which when expressedforms a truncated PMS2 protein; growing the plant; and testing the plantto determine whether the gene of interest harbors a mutation.
 8. Themethod of claim 7 wherein the step of testing comprises analyzing anucleotide sequence of the gene of interest.
 9. The method of claim 7wherein the step of testing comprises analyzing mRNA transcribed fromthe gene of interest.
 10. The method of claim 7 wherein thepolynucleotide encodes human PMS2-134.
 11. The method of claim 10wherein the step of testing comprises analyzing a nucleotide sequence ofthe gene of interest.
 12. The method of claim 10 wherein the step oftesting comprises analyzing mRNA transcribed from the gene of interest.